Code Division Multiple Access (CDMA) signaling is used extensively in cellular phone systems, and in the Global Positioning System (GPS). CDMA signals, in their true form, use the same center frequency, but form individual identifying codes based on each user's phase modulation. The particular pattern of this phase modulation is determined by a set of orthogonal or nearly orthogonal codes. In order to acquire an individual signal, the code for that signal must be used to correlate with the received signal. In this way, several information-transmitting entities can operate within the same radio-frequency band, and an information-receiving station can distinguish between the different entities.
A good example of the application of CDMA is with the GPS, where binary-phase-shift keying (BPSK) is used as the base modulation to create the individual signal codes. In this application, one phase of the carrier would represent a “1,” and a phase 180° from the first phase would represent a “0.” Contiguous phase shifts create a binary code that, when correlated with a copy of itself, allow for the code to be separated from other codes embedded in the RF bandwidth. One implementation of the correlation is to sample and quantize the received signal to form an array of sampled values that contain a code or codes. To determine if a particular code is present, that code is first represented as a sampled RF signal, and then correlated against the received signal. If the sampled, received signal contains the particular code, a correlation peak is produced, which could be either positive or negative. The polarity of the correlation provides the phase modulation that transmits the true information. Thus, the code modulation is the conduit for the true information.
Conveying information through code modulation allows the effective transmission of information when signal-to-noise ratios are very small. This is often the case with modulated reflectance communications, which experience an attenuation of signal strength proportional to the inverse of the distance to the fourth power (A∝1/D4), where distance is between the information source and the information receiver. Conventional RF communications, in which the information source transmits RF power, experience attenuation proportional to the inverse of the distance squared (A∝1/D2). Modulated reflectors, when illuminated by RF energy, will reflect the energy back toward the RF source, and do it in such a way as to modulate the signal for the CDMA process. See U.S. Pat. No. 6,434,372 B1, issued Aug. 13, 2002, to Neagley et al. All patents referred to herein are included herein in their entirety for all purposes.
Modulated reflectors inherently are capable of producing BPSK modulation. The wave reflected from the apparatus can have either a 0° phase shift, or a 180° phase shift depending on the impedance seen by the wave. The usual modulated reflectance apparatus has an antenna of known characteristic terminated with a mismatched impedance, such as an open circuit or a short circuit. Using this knowledge, a code modulation can be implemented by forcing the impedance match to be either an open or a short, depending on the code associated with CDMA signaling.